Ferrite-martensite Dual Phase Research Articles

Effect of prior austenite carbon partitioning on martensite hardening variation in a low alloy ferrite–martensite dual phase steel

Abstract The aim of this research work is to investigate in detail the carbon partitioning within prior austenite developed during austenite to ferrite phase transformation, and consequently its relation to the martensite hardening variation in a low alloy ferrite–martensite dual phase (DP) steel. For this purpose, a wide variety of ferrite–martensite DP samples with different volume fractions of ferrite and martensite have been developed using step quenching heat treatment processes at 600 °C for various holding times after being austenitized at 860 °C for 60 min. Both spot and line scan energy dispersive X-ray spectroscopy for carbon analyses have been used in conjunction with nanoindentation tests to follow the variation of carbon partitioning within prior austenite areas and consequently the associated martensite hardening response in the DP specimens. Experimental results showed that the martensite hardening behavior was quite variable in the ferrite–martensite DP samples and even within a specific martensite area within a specific DP microstructure. A higher level and also a more scattered nanohardness were observed for martensite in the DP samples treated at 600 °C for longer holding times. These results were rationalized due to the variation of carbon partitioning within the prior austenite area developed during isothermal holding in the ferrite–austenite DP region. Longer isothermal holding times were associated with more carbon redistribution within prior austenite as a consequence of more ferrite formation, which resulted in the formation of harder martensite with a more scattered hardening response. Furthermore, compared to the central locations of martensite area, those nearer to the ferrite–martensite interfaces contained higher carbon concentration and consequently higher hardening responses.

Damage mechanism and modeling of void nucleation process in a ferrite–martensite dual phase steel

Damage initiation and growth behavior in high strength dual phase sheet steel, i.e. DP780, was investigated using smooth and notched tensile specimens. By SEM microstructural analyses of pulled and sectioned tensile specimens, void initiation and growth behavior were quantified. Finally, a simple model was proposed to predict the void nucleation kinetics for both kinds of specimens. The predicted values were in good agreement with the experimental data using the model.

Strain localization and damage in dual phase steels investigated by coupled in-situ deformation experiments and crystal plasticity simulations

Ferritic–martensitic dual phase (DP) steels deform spatially in a highly heterogeneous manner, i.e. with strong strain and stress partitioning at the micro-scale. Such heterogeneity in local strain evolution leads in turn to a spatially heterogeneous damage distribution, and thus, plays an important role in the process of damage inheritance and fracture. To understand and improve DP steels, it is important to identify connections between the observed strain and damage heterogeneity and the underlying microstructural parameters, e.g. ferrite grain size, martensite distribution, martensite fraction, etc. In this work we pursue this aim by conducting in-situ deformation experiments on two different DP steel grades, employing two different microscopic-digital image correlation (μDIC) techniques to achieve microstructural strain maps of representative statistics and high-resolution. The resulting local strain maps are analyzed in connection to the observed damage incidents (identified by image post-processing) and to local stress maps (obtained from crystal plasticity (CP) simulations of the same microstructural area). The results reveal that plasticity is typically initiated within “hot zones” with larger ferritic grains and lower local martensite fraction. With increasing global deformation, damage incidents are most often observed in the boundary of such highly plastified zones. High-resolution μDIC and the corresponding CP simulations reveal the importance of martensite dispersion: zones with bulky martensite are more susceptible to macroscopic localization before the full strain hardening capacity of the material is consumed. Overall, the presented joint analysis establishes an integrated computational materials engineering (ICME) approach for designing advanced DP steels.

Effects of microstructure on fatigue crack growth behavior in cold-rolled dual phase steels

Fatigue crack growth behaviors of cold-rolled dual phase steels with different microstructures were investigated at room temperature. The ferrite–martensite dual-phase microstructure was obtained by intercritical annealing. Fatigue crack growth (FCG) behaviors were described by both the Paris model and a new exponential model; fatigue fractography and surface morphology near the fracture were arrested by scanning electron microscopy (SEM); the relationship between macroscopic and microcosmic FCG rate was analyzed quantificationally. The results showed that both the models can be used to describe the fatigue crack growth rate of the samples rather well; fatigue striations and secondary cracks were observed in the fracture surface at stable expanding region (II), while the fracture at rapid expanding region (III) combined dimple and quasi-cleavage morphology; the roughness of fracture surface and the degree of secondary cracking increased with an increase in martensite content, leading to a higher threshold value. Moreover, the changes of microcosmic FCG rate were smoother than that of the macroscopic FCG rate.

Comparative study of eddy current and Barkhausen noise nondestructive testing methods in microstructural examination of ferrite–martensite dual-phase steel

The magnetic properties of ferrite–martensite dual-phase steels were evaluated using eddy current and Barkhausen noise nondestructive testing methods and correlated with their microstructural changes. Several routes were used to produce different microstructures of dual-phase steels. The first route was different heat treatments in γ region to vary the ferrite grain size (from 9.47 to 11.12 in ASTM number), and the second one was variation in intercritical annealing temperatures (from 750 to 890°C) in order to produce different percentages of martensite in dual-phase microstructure. The results concerning magnetic Barkhausen noise are discussed in terms of height, position and shape of Barkhausen noise profiles, taking into account two main aspects: ferrite grain size, and different percentages of martensite. Then, eddy current testing was used to study the mentioned microstructural changes by detection of impedance variations. The obtained results show that microstructural changes have a noticeable effect on the magnetic properties of dual-phase steels. The results reveal that both magnetic methods have a high potential to be used as a reliable nondestructive tool to detect and monitor microstructural changes occurring during manufacturing of dual-phase steels.

Ferrite hardening response in a low alloy ferrite–martensite dual phase steel

This paper is concerned to investigate in details the variation of ferrite hardening response in conjunction with carbon partitioning developed within ferrite during austenite to ferrite phase transformation in a low alloy ferrite–martensite dual phase (DP) steel. For this purpose, a wide variety of ferrite–martensite DP microstructures consisting different volume fractions of ferrite and martensite have been prepared using step quenching heat treatment processes at isothermal temperature of 600°C for various holding times. Nanoindentation measurements have been supplemented by energy dispersive X-ray and microprobe wavelength-dispersive spectroscopic analyses to follow the variation of ferrite hardening response and its relation to the carbon concentration of ferrite in the ferrite–martensite DP microstructures. The experimental results showed that the ferrite hardening response is quite variable depending on the progress of ferrite formation in the ferrite–martensite DP microstructures. For a specific ferrite grain in a specific ferrite–martensite DP microstructure, the location nearer to the ferrite–martensite interfaces has been accompanied with a significant higher carbon concentration and simultaneously higher ferrite hardening response in comparison to that of central regions of ferrite grains. These results are rationalized with a higher concentration of carbon within ferrite developed as a consequence of higher carbon entrapment within defected ferrite area generated at early stage of austenite to ferrite phase transformation at more pre-existing defected area of prior austenite grain boundaries, and that the carbon diluted ferrite region is related to the rejection of carbon from interior ferrite area to the ferrite/austenite interfaces developed on the subsequent isothermal holding times.

Improvement of mechanical properties in a dual-phase ferrite–martensite AISI4140 steel under tough-strong ferrite formation

This paper has been concerned to investigate in details the mechanical properties of AISI4140 heat treatable steel under ferrite–martensite dual-phase (DP) microstructures in conjunction with that of conventional quench-tempered (CQT) full martensitic condition. For this purpose, a wide variety of ferrite–martensite DP samples containing different volume fractions of ferrite and martensite microphases have been developed using step quenching heat treatment processes at 600°C for 20–55s holding times with the subsequent hot oil quenching after being austenitized at 860°C for 60min in the same situation as to the CQT condition. The finalized tempering heat treatment has been carried out at 600°C for 30min for both of direct quenched full martensitic and DP samples in order to optimize the strength–ductility combination. Light and electron microscopes have been used in conjunction with mechanical tests to assess the structure–property relationships in the DP and CQT heat treated samples. The experimental results indicate that the DP microstructures consisting about 7% volume fraction of fine grain boundary ferrite in the vicinity of martensite are associated with excellent mechanical properties in comparison to that of CQT condition. These observations are rationalized in terms of higher carbon concentration of the remaining metastable austenite leading to the harder martensite formation on the subsequent hot oil quenching, and so developing much harder ferrite grains as a consequence of more constraints induced in the ferrite grains during martensitic phase transformation in the remaining austenite adjacent to the ferrite area. The higher martensite volume fraction in the vicinity of thin continuous grain boundary ferrite network has been associated with the harder ferrite formation, causing higher work hardening behavior in the short time treated DP samples. Moreover, it has been found that in order to optimize the mechanical properties of ferrite–martensite DP samples, two independently parameters should be simultaneously controlled: one is the ferrite volume fraction and the other is ferrite morphology.

Prediction of Ferrite-Martensite Dual-Phase Steels Mechanical Properties by Use of Artificial Neural Networks

This paper presents an investigation of the capabilities of artificial neural networks (ANN) in predicting some mechanical properties of Ferrite-Martensite dual-phase steels applicable for different industries like auto-making. Using ANNs instead of different destructive and non-destructive tests to determine the material properties, reduces costs and reduces the need for special testing facilities. Networks were trained with use of a back propagation (BP) error algorithm. In order to provide data for training the ANNs, mechanical properties, inter-critical annealing temperature and information about the microstructures of many specimens were examined. After the ANNs were trained, the four parameters of yield stress, ultimate tensile stress, total elongation and the work hardening exponent were simulated. Finally a comparison of the predicted and experimental values indicates that the results obtained from the given input data reveal a good ability of the well-trained ANN to predict the described mechanical properties.

Phase Analysis on Dual-Phase Steel Using Band Slope of Electron Backscatter Diffraction Pattern

A quantitative and automated phase analysis of dual-phase (DP) steel using electron backscatter diffraction (EBSD) was attempted. A ferrite-martensite DP microstructure was produced by intercritical annealing and quenching. An EBSD map of the microstructure was obtained and post-processed for phase discrimination. Band slope (BS), which was a measure of pattern quality, exhibited much stronger phase contrast than another conventional one, band contrast. Owing to high sensitivity to lattice defect and little orientation dependence, BS provided handiness in finding a threshold for phase discrimination. Its grain average gave a superior result on the discrimination and volume fraction measurement of the constituent phases in the DP steel.

Effect of microstructure on mechanical properties of dual phase steels in the presence of stress concentrators

The study investigated the effect of intercritical heat treatment on mechanical properties of three hot rolled steels with carbon contents ranging from 0.06 to 0.18wt%. The steels were heat-treated at temperatures ranging from 740 to 820°C, followed by water quenching (in some cases air cooling). The primary objective of this study was to investigate the relationship between microstructure and the effect of stress concentrator on deformation and fracture behavior of ferrite–martensite dual-phase (DP) steels. Besides the tensile tests with smooth specimens, notched tensile specimens with two notch geometries (flat rectangular specimens with a central hole and circumferentially notched cylindrical specimens) were used in order to modify the stress triaxiality. Moreover, Charpy impact test was carried out on sub-size specimens (10mm×2.5mm×55mm) with notch root radius of 0.25mm and on pre-cracked specimens. It was found that with increasing martensite volume fraction (MVF) the capacity of the steels to absorb energy (expressed as the product of the ultimate tensile strength and uniform elongation) remains roughly stable or decreases in case of smooth tensile specimens while it always increases in case of specimens with stress concentrator. The experimental results presented in this study clearly showed that DP steels with an excellent combination of strength and uniform or total elongation may have poor energy absorption capacity in the presence of stress concentrators.

Reliability/unreliability of mixture rule in a low alloy ferrite–martensite dual phase steel

Abstract The aim of this paper is to investigate in details the relationship between the volume fractions of ferrite and martensite with the variation of hardness in a low alloy ferrite–martensite dual phase (DP) steel. For this purpose, a wide variety of ferrite–martensite DP samples consisting different volume fractions of ferrite and martensite have been developed using step quenching heat treatment cycle involving reheating at 860 °C for 60 min, soaking at 600 °C salt bath for various holding times followed by 70 °C hot oil quenching. Optical microscopy has been supplemented by electron microscopy and hardness measurements to follow microstructural changes and their relation to the variation in hardness. The results showed that there is a non-linear relationship between the hardness of DP samples with the volume fraction of phase constituents indicating that the mixture rule is not reliable in the ferrite–martensite DP microstructures. The unreliability of mixture rule is related to the variation of ferrite and martensite hardening responses developed in the DP samples. The DP microstructure consisting 6–7% volume fraction of continuous grain boundary ferrite in the vicinity of martensite has been associated with a remarkable higher hardness for both ferrite and martensite in comparison with the other DP microstructures. The higher martensite hardness is due to the higher carbon content of the remaining metastable austenite developed in the ferrite–austenite two phase field area, leading to the harder martensite formation on the subsequent 70 °C hot oil quenching. The harder ferrite grains have been developed as a consequence of more constraints induced in the ferrite grains during martensitic phase transformation. The higher martensite volume fraction in the vicinity of thinner continuous grain boundary ferrite networks has been associated with the harder ferrite formation.

冷延焼鈍後のフェライト−マルテンサイト二相組織鋼中のマルテンサイトの結晶学的組織解析

Cold-rolled and annealed ferrite-martensite dual phase (DP) steel sheets are a useful material for automotive applications because of their excellent balance of strength and ductility. Although few research papers have examined the crystallography and microstructure of lath martensite in DP steel, the characteristics of lath martensite have been investigated in single-phase martensitic steel. In the present study, the crystallography and microstructure of lath martensite in a ferrite-martensite dual phase were studied using electron microscopy and electron diffraction analysis, and the crystallographic orientation relationship between the ferrite and martensite was analyzed. The main results are as follows: (1) The lath martensite in DP steel consists of some number of packets, and these packets consist of blocks. This structure is the same as the microstructure of single-phase lath martensite. (2) The habit planes of the martensite laths in a packet tend to be parallel to the close-packed plane of the adjacent ferrite grain, whose fraction is about 30%.

On the Practice of the Microstructure Adjustment to Improve the Strength-Plastic Comprehensive Performance of a C-Si-Mn Alloyed High-Strength Steel

Four continuous annealing cycles were adopted to get four kinds of multi-phase steels using 0.2C-1.5Si-1.8Mn [wt%] alloyed raw steel and the microstructures and mechanical properties were investigated. The results show that traditional ferrite-martensite dual phase steel has a tensile strength of more than 1000MPa, but has the lowest strength-plastic product of only 18GPa%. The traditional ferrite, bainite and residual austenite TRIP steel has the highest elongation of 31% and hardening exponent of 0.24, but its tensile strength is just over 800MPa. Introducing hard matrix such as acicular bainitc ferrite or martensite to replace the polygonal ferrite matrix, meanwhile introducing metastable austenite, is conducive to obtain high strength and good ductility. Those kinds of modified TRIP steel with hard matrix can reach above 20GPa% of the strength-plastic product on the 1000MPa grade.

Influence of aluminium additions upon CCT diagrams for hot rolled TRIP assisted dual phase steel

The influence of aluminium upon continuous cooling transformation (CCT) diagrams for hot rolled ferrite–martensite dual phase steel is investigated, with continuous cooling experiments performed through the use of dilatometry. Results indicate that additions of aluminium significantly raise the Ar3 and Ar1 temperatures accounting for the increased polygonal ferrite stabilisation during cooling. Acceleration of ferrite transformation is observed, with aluminium displacing both ferrite and pearlite transformations to the left of the CCT diagram. Aluminium is also seen to suppress bainite formation and decrease the martensite start temperature by increasing the hardenability within austenite grains.

Monitoring of the deformation and fracture process of dual phase steels employing acoustic emission techniques

Abstract In this paper, continuing our previous works, a new approach for detection of fracture micro mechanisms of ferrite–martensite dual-phase steels (DPSs) with various microstructures was investigated. For this purpose, dual phase steels with different volume fractions of martensite (VM) were produced by various heat treatment methods on a low carbon steel (0.1% C), and acoustic emission (AE) monitoring was then used during tensile testing of these DPSs. The AE signals from a tensile test using DPS in the range of 12–73% VM and various morphologies, like equiaxed or fibrous martensite phase, were captured. Principally, to understand the AE response and behavior of the martensite or ferrite phase separately, some samples of martensite and heat treated ferrite were tested. After the tests, by utilizing a new function named “sentry function”, we tried to relate the AE signals to various failure mechanisms of these steels. In confirmation of our earlier works, the results show that AE monitoring and sentry function are efficient tools to detect failure micromechanisms, consisting of ferrite–martensite interface decohesion and/or martensite phase fracture, identifying the correlation of failure mechanisms to microstructure in DPS. The results were verified with scanning electron microscopic observations and they indicate that AE monitoring is an efficient tool to detect micromechanisms identifying failure in DPSs.

Effect of initial microstructures on the properties of Ferrite-Martensite Dual-Phase pipeline steels with Strain-Based design

This study aims to investigate the effect of initial microstructures on the properties of ferrite-martensite dual-phase pipeline steels with strain-based design. For this purpose, the as-received acicular ferrite steels were first austenitized at 920 ºC for 15 minutes followed by air cooling and water quenching to produce ferrite-pearlite and ferrite-martensite microstructure, respectively. Subsequently, the steels with ferrite-pearlite, ferrite-martensite and as-received acicular ferrite microstructure were intercritically annealed at 820 ºC for 10 minutes followed by water quenching to produce three different ferrite-martensite dual-phase microstructures. Tensile tests, Vickers hardness and Charpy impact tests were carried out to investigate the mechanical properties. Scanning electron microscope was used to analyze the microstructures and tensile fractographs. The results showed that all the tensile specimens of these three different ferrite-martensite dual-phase steels fractured in ductile mode, however, their microstructures and mechanical properties varied significantly. By contrast, the ferrite-martensite dual-phase steel derived from acicular ferrite initial microstructure had optimal combination of the strength, toughness and deformability, which provided a good candidate for the pipeline steels with strain-based design used in severe geological environments.

Connectivity of slip systems between different phases in ferrite–martensite dual-phase steels

► The crystallographic study of ferrite and martensite in dual-phase was performed. ► Crystal orientation relationship was examined between martensite and ferrite. ► The parallel relationship depends on the heat-treatments of the specimens. ► They suggest the formation of martensite laths. Ferrite–martensite dual-phase high-strength steels exhibit good formability. This study focuses on the morphology and crystallography of martensite and ferrite in these steels by using quantitative analysis. Three kinds of ferrite–martensite dual-phase specimens subjected to three different heat treatments, step-quenched, hot-rolled–annealed–quenched and quenched–annealed–quenched, were used as specimens in the experiments. Martensite in hot-rolled–annealed–quenched specimens exhibits a parallel relationship between the common close-packed planes of martensite packets and the close-packed planes of adjacent ferrites. This is because these specimens contain fine film-like austenite regions. Fine grained austenite contains high elastic strain energy. Martensite laths, whose shear directions are parallel to their habit planes, are formed from fine austenite regions to accommodate martensitic transformation strain induced by emitting dislocations into adjacent ferrite grains.

高強度複合組織鋼板の曲げ変形挙動

The individual deformation behavior of ferrite and martensite was investigated microscopically by using nanoindentation technique to clarify bendability of dual-phase steel sheets. The cold-rolled steel sheets with composition of Fe-0.16mass%C-1.0mass%Si-1.5mass%Mn were annealed at 775°C for 600 s, water-quenched and tempered at 350°C for 300 s. V-shape bending tests with bending angle of 90° were performed for annealed specimens with the thickness of 1.0 mm. Nanoindentation experiments were performed with several peak loads to measure the distribution of the hardness in sub-micron areas, using a cube corner type indenter. The microstructure of the specimen obtained by the heat treatment was ferrite-martensite dual-phase. Ferrite was the dominant microstructure with a volume fraction of approximately 65 %. The nanohardness for ferrite and martensite was 3.2 GPa and 5.9 GPa in the annealed specimen, respectively. Both ferrite and martensite were hardened in the specimen with a bending radius of 1.0 mm. Especially, ferrite was hardened locally in the specimen. The nanohardness for ferrite in the markedly hardened area was 4.0 GPa and it was 3.6 GPa in another area. A distinct plastic flow such as a shear band was observed near the markedly hardened area. Voids nucleated on the ferrite side in the area. It was concluded that the development of shear bands during bending deformation with a small radius caused the local and remarkable work-hardening and that a larger partition of strain into ferrite than martensite domain resulted in the nucleation of micro voids in ferrite.

Hydrogen Embrittlement Resistance Evaluation of Ultra High Strength Steel Sheets for Automobiles

When ultra high strength steel (UHSS) sheets with tensile strength over 980 MPa are applied in automobiles, there is a risk that a type of hydrogen embrittlement fracture called delayed fracture may occur while a vehicle is in use. This paper summarizes the effects of stress, strain, diffusible hydrogen content and the forming mode on the hydrogen embrittlement resistance of UHSS sheets for automotive applications. In this study, 1180 MPa grade ferrite-martensite dual phase steel was used. This material was evaluated by the U-bending and drawn cup methods. It was concluded that high strain, high stress and high diffusible hydrogen content reduced hydrogen embrittlement resistance.In addition, an improved immersion-type hydrogen charging method using an ammonium thiocyanate (NH4SCN) aqueous solution was introduced in this paper. The NH4SCN solution enables control of the diffusible hydrogen content from low to high concentrations using the NH4SCN concentration, and dissolution of the specimens during immersion in NH4SCN was minimal, making it possible to maintain substantially the same surface condition as before immersion.

Effects of Ferrite Grain Size on Dynamic Deformation Behavior of Ferrite-martensite Dual Phase Steel DP980

Effects of ferrite grain size on quasi-static and dynamic deformation behavior of ferrite-martensite dual phase steel(DP980) are investigated.Two groups of experimental dual phase DP steels with the same martensite volume fraction and different ferrite grain size are obtained through performing the experiment with different austenitizing temperature.The quasi-static tensile experiment and the dynamic tensile experiment for the two groups of DP steels are carried out at strain rates ranging from 10-4 to 1 750 s-1 at room temperature using universal testing machine CMT4105 and split Hopkinson tensile bar(SHTB),respectively.Then the true stress-effective plastic strain curves of DP980 steel for five strain rates have been obtained through the data processing.Johnson-Cook(J-C) model,which is not concerned with effects of grain size,and modified Khan,Huang and Liang(KHL) model,which is concerned with effects of grain size,are used to research dynamic deformation behavior of dual phase steel.And coefficient of determination R2 is introduced to determine goodness of fit for experimental results and models.The analysis results of the experiment indicate that modified KHL model is in good agreement with the experimental results,whose R2 reaches 0.998 7,and that modified KHL model has good characterization of deformation behavior for DP980 over a wide range of strain rates,and can reflect effects of grain size on dynamic deformation behavior of DP980.